The removal of solid particulate materials from fluids, both liquids and gases, is customarily accomplished by means of filters. Most commonly, the filters are fabricated from porous solid articles or masses that have a multitude of apertures or pores extending therethrough, which pores may be interconnected and which have predetermined small cross sections such that the fluids flow through the filters from their inlet surface to their outlet surface, but the size of the pores restrains passage of most or all of the particulates, as desired.
The fabrication of a solid filter body utilizing a honeycomb structure consisting of a matrix of intersecting, thin, porous walls which extend across and between two of its opposing end faces to form a plurality of adjoining hollow passages or cells that likewise extend between and are open at the end faces of the structure is well-recognized in the literature. In constructing the filter, one end of each of the cells is plugged or sealed, a first subset of cells being sealed at one end face and the second subset of cells being sealed at the opposing end face of the structure. The particulate-containing fluid is introduced into the inlet face of the structure via those cells having an open end at the inlet face. Inasmuch as those cells are plugged at the opposite end face of the structure, the fluid is forced through the thin, porous walls into the second subset of cells, i.e., those cells which are sealed at the inlet face of the structure but are open at the outlet face thereof. The solid particulates having diameters too great to pass through the pores or connecting passages of the cell walls are collected on the cell walls and the cleansed fluid emerges through the outlet cells.
As can be appreciated, the crux of the fabrication of filter elements from honeycomb structures resides in the plugging of a selected subset of cells while leaving adjacent cells open. Various methods have been proposed for accomplishing that objective. For example:
U.S. application Ser. No. 165,646, filed July 3, 1980 by Rodney I. Frost and Irwin M. Lachman under the title FILTER AND RELATED APPARATUS, discloses the manufacture of filter units from honeycomb structures wherein the cells have transverse, cross-sectional geometries with no internal angles less than 30.degree., such as squares, rectangles, equilateral and certain other triangles, circles, ellipses, etc., and are arranged in mutually parallel rows and/or columns. Alternate cells at one end face are filled in a checkerboard pattern and the remaining alternate cells are plugged in the opposite end face of the structure in a reverse pattern.
Frost and Lachman describe preparing such filter units by manifolding, i.e., plugging or sealing, the end of each cell individually with a hand-held, single nozzle, air-actuated gun. As can be appreciated, such hand plugging of individual cells is not practical for the commercial production of filter units where the selective sealing of hundreds of cells may be required. They also conjectured the use of an array of nozzles so that the sealant could be injected simultaneously in a group or all of the alternate cells at each end face of the honeycomb structure. No design or description of such an array was provided, however.
U.S. application Ser. No. 283,733, filed July 15, 1981 by Rodney I. Frost and Robert J. Paisley under the title IMPROVED METHOD AND APPARATUS FOR SELECTIVELY CHARGING HONEYCOMB STRUCTURES, discloses the selective manifolding of honeycomb structures in the fabrication of filter elements through the use of a mask comprising a rigid plate having a number of apertures extending therethrough to register with the open ends of alternate cells.
Masks for manifolding cells which are regularly interspersed among substantially mutually parallel rows and substantially mutually parallel columns at an open surface of a honeycomb structure have been formed by applying strips of an adhesively-backed flexible webbing impermeable to the sealant, such as masking tape, over selected rows or columns of cells. By covering alternate rows and alternate columns of cells with such strips, the open ends of one-half of a subset of cells arranged in a checkerboard pattern across an end face are exposed. After filling the ends of those cells, the strips are removed and strips applied covering the remaining alternate rows and columns, thereby exposing the open ends of the remaining half of the subset of cells of the checkerboard pattern at the end face for filling.
That method embodiment obviously provides greater flexibility in dealing with any surface height variations present in the face of a honeycomb structure and improved masking of the cell ends not to be sealed, including those that may be damaged, then does the use of a mask in the form of a rigid plate. Nevertheless, the method is not commercially attractive since the individual application of the tape strips is a time consuming task.
In another embodiment, a foraminous rigid plate, wherein the holes registered with alternate cells of a honeycomb structure, was fashioned in such manner that a short filling tube was fitted into each hole which protruded from the face of the plate and into a cell when the plate is aligned over the open cell ends of a honeycomb structure. A sealant was introduced from the opposing face of the plate through the holes into the cell ends receiving the tubes. The filling tubes helped to align the metal plate with the cells to be plugged and reduced the possibility of the sealant being fed into the remaining cell ends covered by the plate.
That embodiment, however, was essentially inflexible, an aspect of substantial significance when the number of cells per unit surface area becomes very high such that the thickness of the cell walls is so reduced that distortions in the locations of cell walls become relatively more severe. Moreover, the rigid construction hazarded damage to brittle, thin-walled honeycomb structures.
In yet another embodiment, rigid rivets were attached at regular intervals along lengths of thin flexible strips which ran along alternate diagonals of cells arranged in mutually parallel rows and mutually parallel columns, such rivets being inserted into and covering the open end of cells along the diagonal. By that action one-half of the cells exposed at an end face of the honeycomb structure were covered in a checkerboard pattern and the open ends of all the remaining cells filled in a single sequence of steps. Whereas the strips containing the rivets exhibited greater flexibility than the plate, they required more handling than was desirable for commercial application.
U.S. application Ser. No. 283,732 filed July 15, 1981 by Roy T. Bonzo under the title IMPROVED METHOD AND RELATED APPARATUS FOR SELECTIVELY MANIFOLDING HONEYCOMB STRUCTURES, now U.S. Pat. No. 4,557,773, discloses manifolding the cells of a honeycomb structure by blocking off the open end faces of the structure with a solid covering applied thereto, said covering preferably being a preformed, transparent polymer film through which holes were formed opposite selected cells at each end face with a suitable tool. Again, however, because of cell distortion, the number of openings which can be formed by each application of the tool is limited.
U.S. Pat. No. 4,411,856 discloses a specially-designed, foraminous flexible mask for selectively manifolding cells of a honeycomb structure. The mask contains a multiplicity of holes which permit passage of a sealant into selected cells, and a multitude of protrusions extending from one surface of the mask which are used to align the mask to the end face of the honeycomb structure, and which extend into and sealably cover the cells which are not to receive the sealant. The holes and protrusions are described as being hand fitted into registry with alternate cells.
U.S. application Ser. No. 283,735, filed July 15, 1981 by Roy T. Bonzo under the title METHOD AND APPARATUS FOR ALIGNING BODY WITH HONEYCOMB STRUCTURE, discloses a method for mechanically aligning, rather than hand-fitting, the mask of U.S. Pat. No. 4,411,856 to an end face of a honeycomb structure. The method generally contemplates approximately centering the mask against an end face of a honeycomb structure and then aligning it through rotational vibration such that the holes therein expose a subset of cells and the protrusions engage the remaining cells.
Bonzo described the use of rigid members extending between the mask and an end face of a honeycomb structure to restrict their relative lateral or lateral and angular movement during the vibration of the mask into alignment. For example:
A pin or like member is inserted through an opening at the axial center of the mask and into the central cell of the end face of the honeycomb structure. In so doing, when the mask is vibrated into alignment, it is free to rotate but is constrained from lateral movement. The number is then withdrawn.
In a modification of that embodiment a rod is passed through an opening at the axial center of the mask and through the length of the central cell of the honeycomb structure. A similar mask is placed over the end of the rod and into contact with the other end face of the structure. Vibration of the masks results in rotational movement, but not lateral movement.
In another embodiment two or more rigid members are inserted through openings in the mask into cells of the end face of the honeycomb structure, which not only restrict lateral movement between the mask and the end face of the structure, but also restrain rotational motion during the vibration action.
U.S. Pat. No. 4,427,728 provides yet another method for selectively manifolding cells of a honeycomb structure for the fabrication of filter units. In like manner to U.S. Pat. No. 4,411,856 above, the subject method contemplates the use of a mask containing a multiplicity of protrusions on one face thereof which extend into a subset of cells. In one embodiment, the protrusions are hollow such that, when the sealant is charged against the mask, it will flow through the protrusions and plug that subset of cells. The mask may be aligned in proper registry utilizing the rotational vibration practice described in Ser. No. 283,735 above.
In a second embodiment, a plurality of solid preformed protrusions or plugs are mounted along thin flexible members at predetermined locations, which plugs are inserted into and cover or block the ends of an equal plurality of cells. When the sealant is charged against the end face of the honeycomb structure, it flows into and plugs the alternate, uncovered cells.
Whereas each of the above disclosures describes means for selectively manifolding the cells of a honeycomb structure for the fabrication of filter bodies, commercial production of such units has demanded efforts to be undertaken to devise even more rapid and efficient methods for carrying out that practice. Furthermore, for certain applications a band of completely sealed cells around the periphery at both ends of the unit is required, resulting in a final composite product comprising a center zone of selectively-manifolding cells to perform as a cellular wall filter and an outside band of fully plugged cells.